1. Field of Invention
The present invention relates to an amplifier apparatus, that is connected to a magneto resistive (MR) head used for reproducing data stored in a magnetic recording and reproducing apparatus, such as a hard disk drive (HDD) or a floppy disk (FDD) drive, and amplifies and outputs signals, that have been read out using the MR head, and also refers to the magnetic recording and reproducing apparatus that includes the amplifier apparatus.
2. Description of Related Art
Conventionally, in a magnetic recording and reproducing apparatus, such as a hard disk drive or a floppy disk drive, in which signals, corresponding to data to be stored, are recorded by magnetizing a magnetic layer in a rotating recording medium and are reproduced by detecting the magnetic state of the magnetic layer, an MR head as a means of reading out signals that have been recorded in order to read out the magnetic state of the magnetic layer.
The MR head is positioned to face the disk-shaped recording medium, a resistance in the MR head changes with changes in the magnetic field in the magnetic layer of the recording medium, and signals are readout by detecting changes in the resistance of the MR head in a form of voltage signals.
Because the changes in the resistance of the MR head are very minute, voltage changes arising from the resistance changes are also minute. Normally, the voltage signals are amplified using an amplifier apparatus consisting of an amplifier circuit, such as shown in FIG. 4, in order to output amplified signals.
In the amplifier circuit, a bias voltage is applied on an MR head Rmr by connecting a first current source I1 and a second current source I2 to a first MR head connection terminal X1 and a second MR head connection terminal X2, respectively, which are connected to both ends of the MR head Rmr, respectively. Changes in the bias voltage, which correspond to the data being read out from the recording medium, are outputted as voltage signals on a first output line L1 and a second output line L2, which are connected to the first MR head connection terminal X1 and the second MR head connection terminal X2, respectively. The first output line L1 and the second output, line L2 are connected to an amplifier A, and the amplifier A generates and outputs amplified signals based on the voltage signals.
More specifically, a first high pass filter H1 for DC cutoff is placed in the middle of the first output line L1, and a second high pass filter H2 for DC cutoff is placed in the middle of the second output line L2. Cutoff frequencies of the high pass filters H1, H2 are 200 kHz to 1 MHz.
Furthermore, in the amplifier circuit, a third high pass filter H3 which is constituted by using a first transistor Q1 instead of a first resistor R1 of the first high pass filter H1, and a fourth high pass filter H4 which is constituted by using a second transistor Q2 instead of the second resistor R2 of the second high pass filter H2, are placed. A first switch SW1 is used for switching operations between a combination of the third high pass filter H3 and the fourth high pass filter and a combination of the first high pass filter H1 and the second high pass filter H2.
The third high pass filter H3 and the fourth high pass filter H4 are used for dissipating in a short period time fluctuations of the voltage signals, which are caused by fluctuations in the bias voltage from a prescribed value in the MR head Rmr, when signals are recorded in the recording medium in the magnetic recording and reproducing apparatus.
Until the voltage signals become stable, the amplifier circuit does not output the amplified signals. After a period of time, that is required for stabilizing the voltage signals, passes, a second switch SW2 on an output side of the amplifier A is turned on for outputting the amplified signals. This time period, during which the second switch SW2 delays starting the output of the amplified signals, is called a delay time.